refs: remove function to read HEAD directly

With the last user of `git_reference__read_head` gone, let's remove it
as it's been reading references without consulting the refdb backends.

src/refs.c

@@ -263,40 +263,6 @@ int git_reference_lookup_resolved(
 	return 0;
 }
 
-int git_reference__read_head(
-	git_reference **out,
-	git_repository *repo,
-	const char *path)
-{
-	git_buf reference = GIT_BUF_INIT;
-	char *name = NULL;
-	int error;
-
-	if ((error = git_futils_readbuffer(&reference, path)) < 0)
-		goto out;
-	git_buf_rtrim(&reference);
-
-	if (git__strncmp(reference.ptr, GIT_SYMREF, strlen(GIT_SYMREF)) == 0) {
-		git_buf_consume(&reference, reference.ptr + strlen(GIT_SYMREF));
-
-		name = git_path_basename(path);
-
-		if ((*out = git_reference__alloc_symbolic(name, reference.ptr)) == NULL) {
-			error = -1;
-			goto out;
-		}
-	} else {
-		if ((error = git_reference_lookup(out, repo, reference.ptr)) < 0)
-			goto out;
-	}
-
-out:
-	git__free(name);
-	git_buf_dispose(&reference);
-
-	return error;
-}
-
 int git_reference_dwim(git_reference **out, git_repository *repo, const char *refname)
 {
 	int error = 0, i;

src/refs.h

@@ -116,24 +116,6 @@ int git_reference_lookup_resolved(
 	const char *name,
 	int max_deref);
 
-/**
- * Read reference from a file.
- *
- * This function will read in the file at `path`. If it is a
- * symref, it will return a new unresolved symbolic reference
- * with the given name pointing to the reference pointed to by
- * the file. If it is not a symbolic reference, it will return
- * the resolved reference.
- *
- * Note that because the refdb is not involved for symbolic references, they
- * won't be owned, hence you should either not make the returned reference
- * 'externally visible', or perform the lookup before returning it to the user.
- */
-int git_reference__read_head(
-	git_reference **out,
-	git_repository *repo,
-	const char *path);
-
 int git_reference__log_signature(git_signature **out, git_repository *repo);
 
 /** Update a reference after a commit. */